A master oscillator may be employed for injection seeding or locking of power amplifiers and oscillators. Applications of such master oscillators include laser radar, laser communications, heterodyne detection, free electron lasers, nonlinear processes, and spectroscopy. Also, pulsed, frequency stable and narrow linewidth lasers are useful for the understanding of noise sources in nonlinear interactions.
A single mode, pulsed laser is often considered to be frequency stabilized. However, mode selection in a pulsed laser is not equivalent to frequency stabilization in that, with any type of mode selection method or device, a long term frequency drift may occur from pulse to pulse. This frequency drift is due to thermal or mechanical disturbances of the optical cavity length. Furthermore, short term frequency variations, or chirping, may occur within one pulse due to rapid refractive index variations of an active gain medium.
Phase conjugation resonators (PCR) have been recognized primarily for the spatial or mode properties of their output beam. Phase conjugation mirrors (PCM) using nearly degenerate four-wave mixing (NDFWM) have also been recognized for their capability of providing a narrow optical bandpass filter. Assuming an adiabatic approximation, pump nondepletion, and weak nonlinear coupling, it is found that as .vertline..kappa..vertline.L increases (here .kappa. is the complex coupling coefficient given by .kappa.=2.pi.f.sub.X.sup.(3) I.sub.p nc; I.sub.p is the intensity of the pump, L is the nonlinear interaction length), the bandpass becomes more sharply peaked at the pumping frequency. When an oscillation condition is reached,, (i.e., .vertline..kappa..vertline.L.about..pi./2) the bandpass approaches zero. The bandpass is then limited only by the linewidth of the pump. It has also been demonstrated that a pulsed laser displays automatic frequency locking using a saturable amplifier without cavity stabilization. Furthermore, frequency chirping is minimized for a PCR if the pulse duration is longer than the cavity round trip time, and the response time of the PCM is shorter than the inverse of the frequency chirping bandwidth of the gain medium.
In a journal article entitled "Pulsed, Frequency Stabile, Narrow Linewidth Master Oscillator" by K. K. Lee, Optics Communications, Vol. 68, No. 4, 15 October 1988 the inventor shows that a phase conjugation oscillator, with one PCM pumped by a frequency stabile, continuous wave (cw) , narrow linewidth laser, together with an active amplitude modulation mode-locker, can provide a frequency stable and narrow linewidth pulse whose properties are nearly identical to the cw pump. A technique is described for introducing frequency modulation. A phase conjugation mirror is described as being comprised of a photorefractive medium, such as BaTiO.sub.3 and GaAs, having short diffusion and recombination times.
In a journal article entitled "Phase Conjugation by Resonantly Enhanced Degenerate Four-Wave Mixing" by R. C. Lind et al., Optical Engineering, March/April 1982, Vol. 21, No. 2, there is described the properties of resonantly enhanced degenerate four-wave mixing (DFWM). The effects of pump absorption and depletion, unequal pump fields, four-level behavior, and motional effects are described.
In a journal article entitled "Nonlinear Optical Phase Conjugation" by D. M. Pepper, optical Engineering, March/April 1982, Vol. 21, No. 2, there is described various aspects of nonlinear optical phase conjugation. In sections (V.1 and V.2) there is described the use of a semiconductor doped glass as a nonlinear medium and the use of a cw PCM (BaTiO.sub.3) to compensate for modal dispersion. Table 1 is a summary of experiments where conjugate reflectivity was measured, the table classifying the PCM results in terms of optical wavelength employed. Of the number of nonlinear mediums listed in the Table, the use of HgCdTe with a CO.sub.2 laser, and InSb with a CO laser, are shown.
An object of this invention is to provide a phase conjugation mirror that provides a maximum nonlinear resonant enhancement for a coherent optical signal incident thereon.
A further object of the invention is to provide a narrow linewidth, continuous wave master oscillator that includes a phase conjugation resonator having a phase conjugation mirror that provides a maximum nonlinear resonant enhancement for a coherent optical signal incident thereon.